HR: 16:30h
AN: V34A-03    [Abstracts]
TI: Lithium Isotopic Composition of Mantle Xenoliths From the Western U.S. - Implications for Metasomatic and Delamination Events of the North American Lithosphere
AU: * Chan, L
EM: glchan@lsu.edu
AF: Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803-4101, United States
AU: Leeman, W P
EM: wleeman@nsf.gov
AF: National Science Foundation, 4201 Wilson Blvd, Arlington, VA 22230, United States
AU: Lee, C A
EM: ctlee@rice.edu
AF: Department of Earth Sciences, Rice University, 6100 Main Street, Houston, TX 77005, United States
AB: We report Li isotopic composition of mantle xenoliths from the central Sierra Nevada to the Colorado Plateau to assess compositional variation of the lithospheric mantle in association with the Cenozoic tectonic events of the western U.S. Xenoliths from the central Sierra Nevada (8.3 Ma Big Creek host lava) are highly enriched in fluid mobile elements (Lee, 2005) with up to 22.6 ppm Li and variably low δ⁷Li in spinel- (1.4 to 3.8‰) and garnet-bearing peridotites (-2.3 and 0.9‰). These characteristics imply that the lithospheric mantle beneath the Sierra has been altered by hydrous fluids. The light isotopic composition (relative to MORB) suggests domination of sediment-derived component in the metasomatic fluids. Spinel peridotite xenoliths from the eastern Sierra (<1 Ma Oak Creek host lava) are not highly enriched in Li (2.7 to 3.8 ppm) and other fluid mobile elements, consistent with little subduction influence. However, clinopyroxenes in Oak creek xenoliths have exceptionally high Li contents (11 to 24 ppm) and are in elemental disequilibrium with coexisting olivine (D^ol/cpx < 0.3). Bulk xenoliths are characterized by light Li isotopic composition (-6.7 and -2.9‰) that possibly reflects isotopic fractionation associated with Li diffusion from the lava into clinopyroxene grains. In the Basin and Range Province, Cima volcanic field mantle xenoliths are not enriched in incompatible elements. Either previous enrichment was stripped away by lithosphere and asthenosphere melting associated with the Basin and Range extension, or the xenoliths sample unmodified asthnospheric mantle. Li, being more compatible, appears to preserve the relatively light isotopic values of -1.5 to 2.6‰. Colorado Plateau xenoliths are characterized by high Th/Nd in addition to a smaller enrichment of fluid mobile elements. Li is variably enriched (2 to 113 ppm) and has normal mantle δ⁷Li (2.6 to 5‰). The isotopic composition correlates with Th/Nd and is consistent with addition of silicate melt. Other xenolith samples (olivine and pyroxene analyses) across the western US (Dish Hill, Lunar Crater, San Carlos, Vulcan's Throne, and Kilbourne Hole) follow the general trend of increasing δ⁷Li from east to west. These Li isotope data support a model of hydration of the Sierran lithosphere during the flat subduction of the Farallon plate and melt metasomatism toward the west as proposed by Lee (2005). We have also analyzed olivine separates from Pliocene lamprophyric basalts in the Central Sierra Nevada (from Farmer et al., 2002). They have 10 to 22 ppm Li and δ⁷Li varies from -11 to 1‰. These light isotopic compositions could be derived from a dehydrated subducted slab with modification by diffusion. Our data support a strongly metasomatized mantle source for the potassic volcanism in the Pliocene which may have been triggered by delamination of the lithosphere. References:
Lee, C.-T.A., 2005. J. Geology 113, 673-685.
Farmer, G.L., Glazner, A.F., Manley, C.R., 2002. GSA Bulletin 114, 754-768.
DE: 1025 Composition of the mantle
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 3621 Mantle processes (1038)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting